Using VSE. Rendered video and now nothing is showing up.
In order for overlays to actually overlay existing video, you need to add an effects strip to tell Blender how to mix it. Generally I use the alpha over strip, but there are other options like alpha under, add, subtract, etc). 

Otherwise, the VSE takes the top most strip and uses that. Without the effect strip all you'll see is the overlay and nothing but black. 

Now if you had some B-roll or stock footage or images that will cover the whole screen, then you wont need a effect strip. Just make sure it's in a higher channel than any other video channel.